Content-Type: multipart/mixed; boundary="==BOUNDARY=="
MIME-Version: 1.0

--==BOUNDARY==
Content-Type: text/cloud-config; charset="us-ascii"

users:
 - blah

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Content-Type: text/x-shellscript; charset="us-ascii"

#!/bin/bash
echo "koz is koz" >> /etc/motd

--==BOUNDARY==--